Looks like I had a different idea of what a periscope should look like!
Couldn’t the “official” version be accomplished with just two wedges joined, then scaled on the z axis?
There is no right or wrong in creating and working with mesh objects. With the skills you have learned, you build the things you need.
But along the course you will learn also the ways to keep your model ‘healthy’. Build object so that you can reuse them, easy in maintenance, low poly, high poly, simulating high poly.
So in a couple of month, when you do this lesson for a second time. You will solve it probably in an more quicker way. But you will do it with the skill-set you have learned. take it step by step.
Good luck and happy meshing.